This invention relates to a device for non-destructive testing of ferromagnetic bodies and to a process for producing values for adjusting the device into an initial state for testing determined by the respective test samples.
A process is known for the non-destructive testing of longitudinally displaceable ferromagnetic bodies for structural faults by magnetizing the respective bodies, on which at least one stationary magnetic field is directed. Structural faults in the bodies cause changes in the course of the magnetic field which are detected by magnetic field detectors arranged resting on or close to the body surface. The magnetic field detectors are arranged in the space between the poles of the respective magnetic field producer in at least one row along the periphery of the body and vertically to the direction of displacement thereof and are each connected by two electrodes in differential connection to inputs of a multiplexer. At the output of the multiplexer is connected a compensation amplifier which is supplied with compensation values according to which one of the magnetic field detectors is selected. Compensation values are necessary due to the uneven size of the magnetic field on the different magnetic field detectors are compensated (DE-PS 31 32 808).
It is an object of the present invention to improve a device of the above-described type such that despite high dynamics of the output signals of the magnetic field detectors, measuring errors due to the position of the respective magnetic field detector and the spread of the parameters of the magnetic field detectors are substantially reduced.
The invention provides a device for the non-destructive testing of longitudinally displaceable ferromagnetic bodies for structural faults by magnetizing a respective body. Structural faults in the bodies are detected by magnetic field detectors arranged resting on, or close to the body surface in free space between poles of a magnet. They are arranged in at least one row along the periphery of the body, vertically to the direction of displacement thereof, and are each connected to a multiplexer. A compensation amplifier is connected to the output of the multiplexer and is supplied with compensation values which are allocated to the individual magnetic field detectors. An equalizer is connected to the compensation amplifier. This equalizer is supplied with a further input with multiplication coefficients to compensate for unequal sensitivities of the detectors and varying intensities of the magnetic field. An analog-digital converter is connected to the output of the equalizer, to which converter an input of adding device is connected. Compensation values which are inputted through the second input of the adder can be supplied to compensate for tolerances of circuit parts of circuits being arranged before the adding device.
Undesirable influences on the measuring accuracy can be substantially removed. The influences result both from the varying intensity of the magnetic field along the row of magnetic field detectors as well as from the spread of the parameters of the magnetic field detectors. In addition, there are the influences on the measuring accuracy caused by the measured value processing members connected one behind the other, which influences can be likewise substantially compensated.
The multiplying equalizer is preferably an analog multiplier. By means of the analog multiplier, an additional amplification of the signals outputted by the compensation amplifier are achieved. Analog multipliers are commercially cheaply available, such that the circuit arrangement can be produced economically. The amplification by the analog multiplier is moreover high so that an additional amplifier behind the analog multiplier can be dispensed with.
In a preferred embodiment, the registers are connected to a processor or are an integral part of a processor. The compensation values supplied by the processor to the registers can be outputted synchronously to the change-over of the inputs of the multiplexer. The processor can also control the change-over of the multiplexer. The compensation values are recorded in the register according to the respective position of the multiplexer.
A process for producing values for adjusting the device according to the invention into an initial state which is ready to test samples comprises the steps of: using a test sample having no structural faults and measuring the initial values of the compensation amplifier for each of the different magnetic field detectors and recording each of said values as compensation values, then using a test sample which contains test faults of predetermined form and size to obtain the initial values of the multiplication coefficients while utilizing the compensation values established in the previous step and recording the multiplication coefficients obtained, and then obtaining the initial values of the adding device by using the test sample having no structural faults and recording the initial values for each of the magnetic field detectors. For the different types and forms of test samples, a fault-free test sample and a test sample provided with predetermined faults are in each case necessary for adjusting the testing device for testing. When the compensation values allocated to a test sample of particular form and type have been determined and recorded, they can be used again in a later test without the testing device having to be readjusted by means of a fault-free test sample and a test sample provided with faults.